The present disclosure is directed to a side registration system and method for accurately registering a sheet in a media handling assembly using forced air. While the registration system particularly relates to the commercial printing of heavy media stock, it is amendable to other types of media.
In media handling assemblies, particularly in printing systems, accurate and reliable registration of the substrate media is desirable as it is transferred in a process direction. In particular, accurate registration of the substrate media, such as a sheet of paper, as it is delivered to an image transfer zone will improve the overall printing process. The sheet is generally conveyed within the system in a process direction. However, often the sheet can shift in a cross-process direction that is lateral to the process direction, or it may acquire an angular orientation—referred to herein as “skew”—meaning that its opposed linear edges are no longer parallel to the process direction. Thus, the sheet can move in three degrees of freedom, which need to be controlled in order to achieve accurate delivery of the sheet.
A slight skew, lateral misalignment or error in the arrival time of the substrate media can lead to errors, such as image and/or color registration errors. Also, the amount of skew can increase or accumulate as the substrate media is transferred between sections of the media handling assembly.
Conventional registration systems steer, straighten and guide sheets in a printing zone so that the images square up and register on the sheet properly. These systems—referred to as differentially driven drive or nip assemblies—employ a series of rollers that push the sheet forward in response to edge sensors that guide the sheet. More specifically, these systems attempt to register sheets by separately varying the speeds of the spaced apart drive rollers, which are controlled in response to sensor measurements to correct for skew mispositioning of the sheet. FIG. 1 shows a conventional sheet registration system according to the PRIOR ART. The registration system 8 consists of two sets of drive nip assemblies 20, 30. Each nip assembly 20, 30 includes a driven wheel 22, 24 (also referred to as drive rolls) and an idler wheel 26, 28, (also referred to as idler rolls) which together engage opposed sides of the sheet S and conveying it within the printing system in a process direction P. Also included are separate drive motors and/or belt assemblies 21, 23 for imparting an angular velocity to the driven wheels 22, 24. The motor may be connected directly to the driven wheels 22, 24 using a common idler shaft 25, although belts 21, 23 are often employed. The registration system 8 also includes sheet leading edge sensors 48, 50, which detect the arrival of a sheet. The sequence of arrival between the different sensors 48, 50 is also used to measure rotational mispositioning (skew) of the sheet. By temporarily driving two motors at slightly different rotational speeds, the conventional registration system provides a slight difference in the total rotation or relative pitch position of each drive roll 22, 24 while the sheet is held in the two nips 20, 30. One side of the sheet is temporarily moved ahead of the other to induce a skew (small partial rotation) in the sheet, i.e., opposite from an initially detected sheet skew in order to eliminate and correct for the detected skew.
FIG. 2 shows another conventional sheet registration system, which also includes two spaced apart nip assemblies 20, 30 and a common idler shaft 25. Skew is corrected by a controller 60 prescribing differentially driven nips 20, 30 for a short period of time while the sheet S is engaged by the nips 20, 30. The sheet arrival time and skew are measured by sensors 48, 50 that are disposed along a sensor line 41 that extends perpendicular to the process direction P. While the nip velocities are varied in these conventional systems, the average velocity between both nips must always equal the desired forward velocity of the sheet in order to maintain process speeds. A difference between the nip velocities will temporarily impart an angular velocity to the sheet used to correct skew.
However, the registration systems implemented in conventional media handling assemblies were designed to process a maximum sheet length of 26-inches. The conventional nip or pinch rollers are not suited for handling larger scale printing jobs that require longer and/or heavy weight stock, s.a., signage, posters, and banners, etc. Existing rollers do not work well on sheets that are longer the current maximum of 26-inches. The rollers end up fighting against each other in a tug-of-war operation. A substantial skew can cause pushing, pulling or shearing forces, which can wrinkle, buckle or even tear the sheet.
Therefore, projects that are printed on heavy weight stock or in larger format require specialized printers that are located at a commercial print shop. A media handling assembly is desired which would allow users to handle commercial printing and signage projects in a noncommercial setting. Accordingly, an improved registration system is desirable for straightening longer and/or heavy weight media, and a method of accurately registering a sheet in a media handling assembly, which overcomes the shortcoming of the prior art.
While the exemplary embodiment is particularly directed to the art of digital image processing, and will be thus described with specific reference thereto, it will be appreciated that the exemplary embodiment may have usefulness in other fields and applications.